Go to a cannon where a Pianta is working in. One is the cannon that leads to Pinna Park. That Pianta needs to be working on the cannon for this to work. Jump on the Pianta and try to use your Ground Pound. Keep doing this process again and again until you go through the cannon. You should be at the bottom of the cannon and under the Pianta, but you aren't able to go to Pinna Park.